In color photography, the dye image is formed during a color developing process. The color developing agent, usually a N,N-dialkyl-p-pheneylenediamine, is oxidized in a reaction with silver halide producing free silver. Oxidized developing agent then reacts rapidly with one or more organic molecules, called couplers, to form dyes. The color of the dye thus formed is determined by the chromophoric structure of the dye contributed by the coupler.
In the preparation of silver halide emulsions for recording colors, it is customary to incorporate a color-forming material in the silver halide emulsion so that upon color development this color-forming compound (coupler) reacts with the oxidation product of the developing agent to form a dye. In a typical emulsion for recording the visible spectrum, the emulsion is coated in several layers, each responsive to a particular region of the spectrum. By suitable control of the development and by use of several couplers, it is possible to form the proper dye in each layer to give an adequate color representation of the light image producing the original exposure, which may be a negative (complement) of the light image or a reversal (positive) of the light image.
Common methods of producing reversal color images include the processes used to develop products, such as those commercially available under the Kodachrome, Ektachrome and Cibachrome trademarks. In processing Kodachrome and Ektachrome products, the color images are produced by first black-and-white developing an exposed photographic element, uniformly exposing or chemically fogging the undeveloped (i.e., residual) silver halide, and finally developing the element in the presence of a color developing agent and a dye-forming coupler. In processing Cibachrome products, the color images are produced by first black-and-white developing a photographic element which already contains the image dyes and then bleaching the dyes in proportion to the developed silver image.
Other methods of producing reversal color images have been described in the prior art. One disclosed in U.S. Pat. No. 3,243,294 relates to a process wherein direct reversal color images are produced by developing an exposed multilayer coupler-incorporated film with a p-phenylenediamine color developing solution in the presence of a black-and-white developing agent and physical development nuclei. In this process, a negative silver image is developed by a black-and-white developing agent which is incorporated in the film. Upon completion of the development of the negative silver image, physical development of the residual silver halide leads to a reversal (i.e., positive) image in color. To obtain the reversal color image, a nucleating agent is required in either the emulsion or the processing solution, and a silver halide solvent is required in the developing solution.
U.S. Pat. No. 3,293,032 and U.S. Pat. No. 3,384,484 describe methods of making positive color images from hydrazones.
U.S. Pat. No. 3,384,484 uses a direct reversal process requiring only a single development step and a bleach-fix step. The direct reversal dye image is formed with certain organic hydrazones that are incorporated in silver halide emulsion layers. In one embodiment, colored hydrazones are used that remain colored in unexposed areas throughout the photographic process, but which are converted into colorless compounds in light-exposed areas during processing. In a second embodiment, a class of colorless hydrazones are used which, during processing, form colorless compounds in light-exposed areas, but in unexposed areas the residual hydrazone is oxidized to a colored dye by a silver bleach step.
Another method is described in U.S. Pat. No. 3,647,452, wherein direct reversal color images are produced by developing an exposed incorporated-coupler (e.g., Ektachrome) film with a p-phenylenediamine color developer in the presence of a competing coupler sufficient to prevent dye formation until the development of the exposed silver is completed. Neither previous black-and-white development (MQ) nor reexposure of the film are necessary. By this method, the film can be processed in a sequence requiring only the two steps of color development and bleach-fixing.
The present invention relates to a novel method capable of producing reversal color images using only black-and-white developing agents. A color development step is unnecessary because a preformed leuco dye is incorporated into the silver halide emulsion. Therefore, the dye molecule does not have to be formed during the development process. The leuco dye need only be oxidized to its corresponding colored form in an imagewise fashion.